The invention relates to a four-wheel vehicle drive system and more specifically to an adaptive vehicle drive system which incrementally shifts from two-wheel drive to four-wheel drive upon sensing certain conditions.
The performance advantages of four-wheel vehicle drive systems are well recognized. Improved vehicle stability while traversing rain soaked or ice or snow covered highways, handling and control on gravel or uneven pavement and simply maintaining traction in off road situations are all readily acknowledged benefits. Concomitant and less desirable attributes of four-wheel drive systems relate to reduced gas mileage from increased drive line friction and increased vehicle weight. Such increased drive line friction occurs in part time four-wheel drive systems which rotationally couple the front and rear vehicle propshafts. Such vehicle weight increases are particularly pronounced if the system is designed with a differential between the front and rear drive shafts for full-time engagement and operation rather than intermittent operation when conditions specifically demand it.
Furthermore, while part time four-wheel drive systems which lock the front and rear propshafts together provide obvious benefits of traction and stability in straight line driving, the disparity between the ground speed at the front wheels and the ground speed at the rear wheels during cornering can itself result in wheel slip and hopping of the vehicle. Thus, allowing the front and rear output shafts of the transfer case to operate at different speeds during cornering is beneficial.
Many four-wheel drive systems employing diverse control and torque distribution strategies have been designed and utilized. These various approaches are embodied in United States patents.
For example, U.S. Pat. No. 4,417,641 teaches an automatic four-wheel drive system having an electromagnetic clutch and steering sensor. When the steering wheels are turned greater than a predetermined angle, the electromagnetic clutch is de-energized and two of the driving wheels are disconnected. The system also includes a slip sensor.
U.S. Pat. No. 4,718,303 is co-owned by the assignee herein. It discloses a transfer case having an electromagnetic ramp clutch which is modulated to adjusted the torque distribution in a full time four-wheel drive system.
In U.S. Pat. No. 4,937,750, a microcomputer compares signals from front and rear axle speed sensors. If the difference is greater than a certain value, a clutch is engaged to interconnect the front and rear prop shafts to prevent slip.
U.S. Pat. No. 4,989,686 discloses a full time four-wheel drive system including wheel slip detectors. The output signal from the wheel slip detectors is utilized to drive a proportional clutch which then delivers torque to whichever axle is rotating more slowly. U.S. Pat. No. 5,002,147 discloses a four-wheel drive system which achieves torque splitting between the front and rear axles. The system utilizes four separate wheel speed sensors as well as a steering angle sensor.
In U.S. Pat. No. 5,060,747, a vehicle torque distribution system is taught which includes means for determining both vehicle speed and the difference between the speeds of the front and rear wheels which indicates wheel slip. The vehicle speed data is utilized to correct the sensed wheel speed difference and this corrected value is then utilized to produce a clutch engagement signal.
U.S. Pat. No. 5,090,510 discloses a four-wheel drive system having a differential and a hydraulic clutch disposed in parallel between the front and rear drive shafts. Engagement of the clutch restricts action of the differential to achieve a desired rotational speed difference between the front and rear wheels.
The foregoing review of relevant United States patents reveals many approaches to controlling torque delivery in four-wheel drive vehicles and certain shortcomings as well. Hence, improvements in the art of four-wheel drive vehicle torque delivery systems are both possible and desirable.
An on demand four-wheel vehicle drive system monitors vehicle performance and operating conditions and controls torque delivery to the vehicle wheels. The vehicle drive system includes a transfer case having primary and secondary output shafts driving primary and secondary axles, a plurality of speed and position sensors and a microcontroller. The speed and position sensors include a vehicle speed sensor which monitors the rotational speed of a wheel of the secondary drive line, a pair of primary and secondary drive line speed sensors, throttle position and steering angle sensors and brake and driveline status sensors. The secondary axle may include coupling components such as locking hubs or an axle disconnect.
The transfer case includes a modulating electromagnetic clutch controlled by the microcontroller which selectively transfers torque from the primary output shaft to the secondary output shaft. A planetary gear assembly or similar device providing high and low speed ranges as well as neutral may also be included in the transfer case.
Selection of the on demand vehicle drive system both provides a minimum (standby) current to the clutch which establishes a minimum torque transfer level and activates the secondary axle engaging components. When the speed of one of the front or rear drive shafts overruns, i.e., exceeds, the speed of the other drive shaft by a predetermined value related to the vehicle speed and the identity of the overrunning shaft, indicating that wheel slip is present, clutch current is incrementally increased to increase clutch engagement and torque transfer to the secondary drive shaft until the speed difference between the drive shafts and thus wheel slip is reduced below the predetermined value. Incremental reduction of the clutch current then occurs. If, as clutch current is reduced, a speed difference exceeding the predetermined value again occurs, indicating that wheel slip is again present, clutch current is increased. Alternatively, if the speed difference and wheel slip remains below the predetermined value, clutch current is reduced until it returns to the minimum current and torque transfer level.
The on demand vehicle drive system may be an active full-time system, may be selectively activated by the vehicle operator or may be automatically activated by driving conditions. The system may be utilized with either primary front wheel or primary rear wheel drive configurations. The clutch may be located anywhere in that portion of the powertrain which delivers torque to the secondary axle, e.g., within the secondary axle differential.
In addition to vehicle speed, the predetermined value is dependent upon whether it is the front or the rear drive shaft (and axles) that is overrunning the other and it may also be dependent upon, for example, steering angle and braking. The clutch may also be incrementally engaged when open engine throttle positions are sensed by the throttle position sensor. The invention offers improvements of lower weight, smaller package size, efficiency, smoothness of operation, fast response and enhanced vehicle handling over current state of the art.
Thus it is an object of the present invention to provide an on demand vehicle drive system which incrementally shifts from primary axle to primary axle and secondary axle torque delivery in response to sensed wheel slip and incrementally returns to primary axle drive when such slip is no longer sensed.
It is a further object of the present invention to provide an on demand vehicle drive system which may be utilized in vehicles having either the front or the rear axle as the primary drive means and the other axle as the secondary drive means which is driven in response to sensed wheel slip.
It is a still further object of the present invention to provide an on demand vehicle drive system which automatically activates and incrementally shifts from two-wheel to four-wheel drive in response to sensed wheel slip.
It is a still further object of the present invention to provide an on demand vehicle drive system which may be manually activated by the vehicle operator and which automatically shifts from two-wheel to four-wheel drive in response to sensed wheel slip.
It is a still further object of the present invention to provide an on demand vehicle drive system which may optionally include a high and low range planetary gear assembly in the transfer case.
It is a still further object of the present invention to provide an on demand vehicle drive system in which the predetermined value of drive shaft overrun causing incremental torque transfer from the primary drive shaft to the secondary drive shaft depends upon vehicle speed and the identity of the overrunning drive shaft.
Further objects and advantages of the present invention will become apparent by reference to the following Description of the Preferred and Alternate Embodiments and appended drawings wherein like reference numerals designate the same components.